It is known to produce optical waveguides in plane-parallel plates, in which waveguides light is guided by total reflection. To this end, structures have to be provided in the plane-parallel plate, which structures have a refractive index which is larger than the refractive index of the material of the plate. It is known to provide such an increased refractive index by causing an ion exchange in the material, through which ions in the material of the plate are replaced by ions having a larger ion radius. The surface of the plate is covered by means of a mask except for those areas in which the refractive index shall be increased. The plate is heated to an increased temperature and brought into contact with appropriate substances. Then an ion exchange takes place and, thus, the refractive index is increased in the areas which are not covered by the mask.
U.S. Pat. No. 3,857,689 describes the production of an integrated optical circuit with which a mask is applied to a plane-parallel plate (substrate), which mask defines the desired optical pattern. Then the plate is brought into a bath of molten salt. This molten salt comprises monovalent positive ions which have a larger influence on the refractive index than the positive ions in the material of the plate. It is ensured that an ion exchange takes place through which positive ions of the plate are replaced by ions of the molten salt. This leads to an increase of the refractive index in the areas near the surface which are not covered by the mask and which are exposed to the molten salt.
From Japanese patent application No. 58-118 609 it is known to support the ion exchange by applying an electrical field. In the method described therein, a mask and then a thin silver layer is applied by vacuum-metallizing to a plane-parallel plate. An anode of chrome is applied to the silver layer. A cathode of aluminum is provided on the opposite side. The plate is heated and, simultaneously, a voltage is applied between the anode and the cathode. By means of the electrical field, silver ions are transported into the material of the plate. Thereby, an optical waveguide having an increased refractive index compared to the plate is produced in the areas which are not covered by the mask.
Furthermore, it is known to hold a plane-parallel plate in a horizontal position through a suction bell which communicates with a vacuum, which plate shall be provided with waveguides. A supply of molten salt is contained in the suction bell. The side of the plane-parallel plate remote from the suction bell is provided with a mask. This side of the plate is plunged into a further supply of the molten salt. An electrical field is applied to the molten salt through electrodes. Also, here, positive ions of a larger diameter diffuse from the lower supply of molten salt into the plate and thereby provide optical waveguides having an increased refractive index in the uncovered areas, while, on the other hand, ions having a smaller diameter emerge from the material of the plate into the molten salt on the upper side of the plate.
Furthermore, it is known to hold a plane-parallel plate vertically. Cups are placed with corresponding sealing surfaces on both sides of the plate such that cavities are formed on both sides of the plate. These cavities are limited by a surface of the plate on a vertical side face and are open at the top. Molten salt is filled into these cavities. The cups carry an anode and a cathode, respectively, such that an electrical field is generated through the molten salt and the plate. Again, the plate carries a mask on the side of the anode.
From the (not pre-published) German Patent Application No. P 38 02 837.9 a device for carrying out field-supported ion exchange in plane-parallel plates is known in which the plates are clamped between two supporting bodies of metal. The supporting bodies form cavities on both sides of the plate, which cavities are open towards the plate. The cavities are surrounded by closed sealing surfaces which engage the plate. Supply vessels for molten salt communicate with the lower ends of the cavities through filling supply conduits. Venting conduits extend from the cavities.
When certain structures are produced, e.g. ring resonator structures, mask areas electrically insulated from each other remain on the glass substrate. It has been found that, in these cases, the formed waveguides have considerable deficiencies.